Long uninterrupted photometric observations of the Wolf-Rayet star EZ CMa by the Toronto {\em{BRITE}} satellite reveal a very fast apsidal motion

Context. The variability of the Wolf-Rayet star EZ CMa has been documented for close to half a century, and a clear periodicity of $\sim$3.7 days is established. However, all attempts to prove that it is a binary have failed because the photometric, spectroscopic, and polarimetric variations are not coherent over more than a few orbital cycles. Aims. In this letter we show that the lack of coherence in the variability can be explained with a very rapid apsidal motion in a binary orbit.} Methods. We measured the times of minima in a recently published exceptionally long photometric light curve obtained by the Toronto {\emph{BRITE}} satellite. The apsidal motion and the system eccentricity are determined from the length of the time intervals between these minima, which alternate in their duration, following a pattern that is clearly associated with apsidal motion. These minima are superposed on brightness enhancements of the emission from a shock zone, which occur at about the times of periastron phases. Results. We determine the orbital periodicity, $P_{a}=3.63\,$d, and the period of the apsidal motion, $U\simeq 100\,$d, which together yield an average sidereal period of $P_{s}=3.77\,$d. The eccentricity is found to be close to 0.1. The rate of periapsis retreat changes significantly over the period of observation and is determined to be $-16^\circ\,\mathrm{P}^{-1}_a$ at the beginning of the observing period and $-10^\circ\,\mathrm{P}^{-1}_a$ at the end. Conclusions. We demonstrate that by introducing a fast apsidal motion, the basic photometric variability is very well explained. The binary nature of EZ CMa is now established. This might imply that other apparently single Wolf-Rayet stars that emit hard X-rays, similar to EZ CMa, are also binaries.Comment: A&A Letter in press, 5 pages, 3 figure

The nature of the companion in the Wolf-Rayet system EZ Canis Majoris

EZ Canis Majoris is a classical Wolf-Rayet star whose binary nature has been debated for decades. It was recently modeled as an eccentric binary with a periodic brightening at periastron of the emission originating in a shock heated zone near the companion. The focus of this paper is to further test the binary model and to constrain the nature of the unseen close companion by searching for emission arising in the shock-heated region. We analyze over 400 high resolution the International Ultraviolet Explorer spectra obtained between 1983 and 1995 and XMM-Newton observations obtained in 2010. The light curve and radial velocity (RV) variations were fit with the eccentric binary model and the orbital elements were constrained. We find RV variations in the primary emission lines with a semi-amplitude K$_1\sim$30 km/s in 1992 and 1995, and a second set of emissions with an anti-phase RV curve with K$_2\sim$150 km/s. The simultaneous model fit to the RVs and the light curve yields the orbital elements for each epoch. Adopting a Wolf-Rayet mass M$_1\sim$20 M$_\odot$ leads to M$_2\sim$3-5 M$_\odot$, which implies that the companion could be a late B-type star. The eccentric (e=0.1) binary model also explains the hard X-ray light curve obtained by XMM-Newton and the fit to these data indicates that the duration of maximum is shorter than the typical exposure times. The anti-phase RV variations of two emission components and the simultaneous fit to the RVs and the light curve are concrete evidence in favor of the binary nature of EZ Canis Majoris. The assumption that the emission from the shock-heated region closely traces the orbit of the companion is less certain, although it is feasible because the companion is significantly heated by the WR radiation field and impacted by the WR wind.Comment: 16 pages, 18 figure

New X-ray Detections of WNL Stars

Previous studies have demonstrated that putatively single nitrogen-type Wolf-Rayet stars (WN stars) without known companions are X-ray sources. However, almost all WN star X-ray detections so far have been of earlier WN2 - WN6 spectral subtypes. Later WN7 - WN9 subtypes (also known as WNL stars) have proved more difficult to detect, an important exception being WR 79a (WN9ha). We present here new X-ray detections of the WNL stars WR 16 (WN8h) and WR 78 (WN7h). These new results, when combined with previous detections, demonstrate that X-ray emission is present in WN stars across the full range of spectral types, including later WNL stars. The two WN8 stars observed to date (WR 16 and WR 40) show unusually low X-ray luminosities (Lx) compared to other WN stars, and it is noteworthy that they also have the lowest terminal wind speeds (v_infty). Existing X-ray detections of about a dozen WN stars reveal a trend of increasing Lx with wind luminosity Lwind = (1/2) M_dot v_infty^2, suggesting that wind kinetic energy may play a key role in establishing X-ray luminosity levels in WN stars.Comment: 20 pages, 5 figure

Modeling of the atmospheric response to a strong decrease of the solar activity

We estimate the consequences of a potential strong decrease of the solar activity using the model simulations of the future driven by pure anthropogenic forcing as well as its combination with different solar activity related factors: total solar irradiance, spectral solar irradiance, energetic electron precipitation, solar protons and galactic cosmic rays. The comparison of the model simulations shows that introduced strong decrease of solar activity can lead to some delay of the ozone recovery and partially compensate greenhouse warming acting in the direction opposite to anthropogenic effects. The model results also show that all considered solar forcings are important in different atmospheric layers and geographical regions. However, in the global scale the solar irradiance variability can be considered as the most important solar forcing. The obtained results constitute probably the upper limit of the possible solar influence. Development of the better constrained set of future solar forcings is necessary to address the problem of future climate and ozone layer with more confidenc

Observations of Binary and Single Wolf-Rayet Stars with XMM-Newton and Chandra

We present an overview of recent X-ray observations of Wolf-Rayet (WR) stars with XMM-Newton and Chandra. A new XMM spectrum of the nearby WN8 + OB binary WR 147 shows hard absorbed X-ray emission, including the Fe K-alpha line complex, characteristic of colliding wind shock sources. In contrast, sensitive observations of four of the closest known single WC (carbon-rich) WR stars have yielded only non-detections. These results tentatively suggest that single WC stars are X-ray quiet. The presence of a companion may thus be an essential factor in elevating the X-ray emission of WC + OB stars to detectable levels.Comment: To appear in conf. proceedings: Close Binaries in the 21st Century - New Opportunities and Challenges, eds. A. Gimenez, E. Guinan, P. Niarchos, S. Rucinski; Astrophys. and Space Sci. (special issue), 2006. 4 pages, 2 figure

Fundamental stellar parameters of zeta Pup and gamma^2 Vel from HIPPARCOS data

We report parallax measurements by the HIPPARCOS satellite of zeta Puppis and gamma^2 Velorum. The distance of zeta Pup is d=429 (+120/ -77) pc, in agreement with the commonly adopted value to Vela OB2. However, a significantly smaller distance is found for the gamma^2 Vel system: d=258 (+41/-31) pc. The total mass of gamma^2 Vel derived from its parallax, the angular size of the semi-major axis as measured with intensity interferometry, and the period is M(WR+O)=29.5 (+/-15.9) Msun. This result favors the orbital solution of Pike et al. (1983) over that of Moffat et al. (1986). The stellar parameters for the O star companion derived from line blanketed non-LTE atmosphere models are: Teff=34000 (+/-1500) K, log L/Lsun=5.3 (+/-0.15) from which an evolutionary mass of M=29 (+/-4) Msun and an age of 4.0 (+0.8/-0.5) Myr is obtained from single star evolutionary models. With non-LTE model calculations including He and C we derive a luminosity log L/Lsun~4.7 (+/-0.2) for the WR star. The mass-luminosity relation of hydrogen-free WR stars implies a mass of M(WR)~5 (+/-1.5) Msun. From our data we favor an age of ~10 Myr for the bulk of the Vela OB2 stars. Evolutionary scenarios for zeta Pup and gamma^2 Vel are discussed in the light of our results.Comment: Submitted to ApJ Letters (misprints corrected

X-ray Emission from Nitrogen-Type Wolf-Rayet Stars

We summarize new X-ray detections of four nitrogen-type Wolf-Rayet (WR) stars obtained in a limited survey aimed at establishing the X-ray properties of WN stars across their full range of spectral subtypes. None of the detected stars is so far known to be a close binary. We report Chandra detections of WR 2 (WN2), WR 18 (WN4), and WR 134 (WN6), and an XMM-Newton detection of WR79a (WN9ha). These observations clearly demonstrate that both WNE and WNL stars are X-ray sources. We also discuss Chandra archive detections of the WN6h stars WR 20b, WR 24, and WR 136 and ROSAT non-detections of WR 16 (WN8h) and WR 78 (WN7h). The X-ray spectra of all WN detections show prominent emission lines and an admixture of cool (kT 2 keV) plasma. The hotter plasma is not predicted by radiative wind shock models and other as yet unidentified mechanisms are at work. Most stars show X-ray absorption in excess of that expected from visual extinction (Av), likely due to their strong winds or cold circumstellar gas. Existing data suggest a falloff in X-ray luminosity toward later WN7-9 subtypes, which have higher Lbol but slower, denser winds than WN2-6 stars. This provides a clue that wind properties may be a more crucial factor in determining emergent X-ray emission levels than bolometric luminosity.Comment: 42 pages, 5 tables, 10 figure